The present invention is related to conical-type diaphragms and particularly to diaphragm piston assemblies in which conical-type diaphragms are employed with a reinforcing fabric, as typically required for high pressure applications.
Generally, diaphragm piston operation requires a convolution in the skirt area of the diaphragm to accommodate axial movement of the piston member, to which the inner periphery of the diaphragm is clamped, relative to a body, in which the outer periphery of the diaphragm is clamped. Diaphragms having a molded-in convolution have given way to conical-type diaphragms, such as shown in FIG. 1 of the drawings. In that the fabric in these conical diaphragms tends to remain centered within the rubber during the molding process, conical diaphragms are better adapted to the application of reinforcing fabric than are diaphragms having a molded-in convolution. Accordingly, the conical diaphragm, as shown in U.S. Pat. No. 3,173,342 provides a more reliable diaphragm over a longer service life.
In that the conical-shaped diaphragm is molded from a flat, annular, preformed blank having a layer of fabric pressed between opposing outer layers of rubber, it will be appreciated that the fabric becomes somewhat distorted when forced to assume a conical shape during molding. This is due to the fact that the warp and woof threads of the fabric material located near the inner diaphragm periphery are compressed more closely together, while the warp and woof threads near the outer diaphragm periphery are spread further apart. While this fabric distortion is insufficient to wrinkle the diaphragm, there is a definite proclivity of the diaphragm to assume a somewhat oval shape, due at least in part to this fabric distortion.
This oval shape of the cone diaphragm is more pronounced at its outer periphery, which is clamped in the valve body during assembly following clamping of the inner diaphragm periphery between a piston and follower. Since a convolution naturally forms in the annular space between the piston and body during assembly, there is a tendency for the outer diaphragm periphery to be flipped out of its clamping groove by the tension in the convolution. This assembly difficulty is exacerbated by the fact that the outer diaphragm periphery, being oval-shaped, does not fit perfectly in the annular clamping groove. In order to overcome this problem, the diameter of the clamping bead at the diaphragm outer periphery is made oversize, so that its interference with the clamping groove O.D. provides an added measure of retention to facilitate assembly. This proved to not be a full-proof solution to the problem, however, due to the fact that the fabric at the diaphragm outer periphery, which is trimmed after completion of the diaphragm molding process, can not be trimmed perfectly flush with the diaphragm bead and may also be trimmed irregularly. Consequently, the desired interference fit between the diaphragm outer periphery and the clamping groove O.D. necessary to assure proper diaphragm retention is not consistently obtained.
In addition, further variations in the O.D. of the conical diaphragm of FIG. 1 occur due to manufacturing tolerances in the mold portions between which the diaphragm cavity is formed. This is due to the fact that the outer clamping bead of the diaphragm is formed across the aforementioned mold portions, as can be seen in FIG. 2 of the drawings.